Understanding Transistor Biasing Techniques

Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS. So in this video, we will learn about the basics of the transistor biasing and we will also learn the different terminologies like Q-point and the load line in the context of the transistor biasing. So, first of all, let's understand what is biasing and why we need to bias the transistor. Now, as you are aware, one of the most common application of the BJT is to use it as an amplifier. Where if we apply the time varying signal as an input, then it amplifies the input signal. But this BJT won't amplify the input signal until we apply the DC power supply. And in fact, The energy supplied by this DC supply is used to amplify the input signal. So, the process of applying this DC voltage source to the BJT is known as the biasing. And if you have followed the previous videos, we had seen that the common emitter configuration of the BJT is frequently used for the signal amplification. So, for understanding the transistor biasing, we will use the common emitter configuration. Now, whenever The BJT is used as an amplifier, then it is biased in such a way that base emitter junction gets forward biased and the collector base junction gets reversed biased. And once we apply the DC voltage sources to the BJT, then the current and the voltage will establish in the circuit. And as we had seen in the previous video, this voltage Vbe and Ib are the parameters on the input side. While this voltage Vce and Ic. are the parameters of the output side. And after biasing the BJT in the active region, if we apply the sine wave as an input, then we should get the amplified signal at the output. But whether we will get the proper amplified output or not, that depends on how well the BJT is biased in the active region. So let's understand it through the output characteristics. So let's say, the BJT is biased in such a way that The VCE is equal to 5V and the collector current IC is equal to 10mA. And this point on this collector curve is known as the operating point or the Q point of the BJT. And the reason it is known as the operating point, because it tells us the operating voltage and the current of the given transistor. So, this particular operating point tells us that the transistor is biased in such a way that, The base current Ib is equal to 30 uA and the voltage Vc and the Ic are 5V and 10 mA respectively. So, basically this point on the collector curve tells us the operating condition of the transistor. Now, as I said earlier, the operating point should be in the active region. But in the active region, it could be set anywhere. So, this point could be in the center or it could be over here. Or maybe it could be over here also. So whenever the operating point is over here and on top of it, whenever we apply the AC signal, then some portion of the amplified signal will get clipped. Because as you can see, this voltage we see cannot go below 0V. Similarly, whenever it is operated at this point, then also some portion of the current will get clipped. Because the collector current current go below 0 A. And due to that, some portion of this voltage we see will also get clipped. That means whenever the operating point is near saturation or the cut-off region, then it may lead to the non-linear distortion in the output waveform. Also, if you notice over here, when the BJT is operating in this region, then it is operating near the breakdown region. So, the operation of the BJT in this region should be avoided. On the other end, when the operating point is in the center of the collector curves, then it is possible to amplify the input signal without any kind of distortion. And also in this region, the gain of the BJT is also almost constant. That means for the small signal amplification, the biasing point or the operating point should be in the center region of the collector curves. Now another important aspect of the biasing is the stability of the operating point. Because with temperature, the operating point may get changed. Because with temperature, the device parameters like the transistor current gain and the reverse saturation current will get changed. So, the designed biasing circuit should provide the temperature stability such that even if there is a change in the temperature, then there is a minimum change in this operating point. And the stability of the operating point is defined by the term stability factor, which indicates the change in the operating point with the change in the temperature. So, in this video as well as in the upcoming videos, we will see the different biasing techniques for the BJT. And we will compare the stability of the different biasing configurations. So, first of all, let's start with the very simple fixed bias configuration. So, this is the circuit of the fixed bias configuration. Or if I redraw the same circuit, then it can be redrawn like this. So, as you can see over here, the AC input signal is applied between the base and the emitter terminal, while the output is taken across this collector and the Now, here as we are interested in the DC analysis of the circuit, so we can replace all the capacitors with the open circuit. And then after if you see the equivalent circuit, then it will look like this. Now, the reason it is known as the fixed-base configuration, because for the given power supplies, if we fix the value of this base resistor, then this base current IB will get fixed. And here. If we mark the voltages, then this voltage VBE is the voltage on the input side, while the voltage VCE is the voltage on the output side. So, first of all, now let's find the expression for this base current IB. So, if you notice over here, the voltage at this node is equal to VBE. So, from this, we can say that this base current IB is equal to this voltage VBB minus VCE. VBE divided by RB. Now, here the typical value of VBE is equal to 0.7V. So, for the given supply voltage, once we set the value of RB, then this base current IB will get fixed. And once we know the value of this base current, then we can find the value of this collector current and the voltage VCE. Now, we know that this collector current IC can be given as... So, from the base current Ib, we can find the value of this collector current. Now, if we apply the KVL in this output side, then we can write the expression as Vcc minus voltage across this resistor Rc, let's say that is equal to minus Vrc minus Vce that is equal to zero. Or from this, we can say that this voltage Vce. is equal to Vcc minus Vrc. That is equal to Vcc minus Ic times Rc. So, in this way, we can also get the value of the Vce. So, here this voltage Vce and the collector current Ic defines the operating point of the transistor. Now, from the equations if you observe, if the value of Rb or Rc changes, or the value of Vcc changes, then the operating point will also change. So now, let's find out how this network parameters defines the possible values of the operating point. Now, from this expression, we can write this collector current Ic is equal to voltage Vcc minus Vce divided by Rc. So, this collector current Ic will be maximum whenever this voltage Vce is equal to That means the maximum value of the collector current will be equal to voltage Vcc divided by Rc. That means this collector current will be maximum whenever the voltage Vce is equal to zero. Similarly, whenever this collector current Ic is zero, at that time the value of Vc will be maximum. So, we can say that voltage Vce max is equal to Vcc. So, from this graph, we can say that whenever the collector current Ic is zero, at that time the value of Vc is maximum. So, if we join these two points, then we will get the possible values of the operating point for the given values of Vcc and the Rc. So, this line is also known as the load line. Because it is defined by the value of this resistor Rc. Now here, if the value of Ib is changed by varying this Rb, then the Q point will also change. So, for the given value of Vcc and Rc, if the value of Ib is increased, then the Q point will also move upwards. Similarly, by keeping the Vcc fixed, if we change the value of Rc, then the load line will also change. So here, three different door lines are shown for the different values of the Rc. And as you can see, for the fixed value of the base current, if the load line changes, then the operating point will also shift towards the left-hand side. And similarly, by keeping the value of Rc fixed, if we change the value of Vcc, then Then the load line will look like this. So here, the different load lines are shown for the different values of Vcc. But let's say, for the given value of Vcc and Rc, we are getting this load line. And the operating point is also set over here. Now as I said earlier, as the temperature changes, then the operating point will also change. Because with temperature, the value of β or the current gain of the transistor will also change. Or in case, if we replace the transistor, then also the value of β will change. Because if you take the similar transistors, then also they have the different values of β. So, now let's take one simple example and let's understand how the variation in this β can affect the operating point. So, let's say for a one transistor, the nominal value of the β is equal to 100. And due to the change in the temperature, or due to the replacement of the transistor, this β varies from 50 to 200. And assuming this voltage VBE is not changing with the temperature, this base current IB is set to 30 uA. Now here, let's say the value of Rc is equal to 1.5 kΩ and the value of Vcc is equal to 10 V. So, whenever the β is equal to 100, at that time, This collector current Ic will be equal to 100 times 30 uA. That is equal to 3 mA. And for that, if you calculate the value of VCE, then it will be equal to 10V-1.5 times 3 mA. That is equal to 5.5V. So, at that time, the operating point would be over here. That means the VCE is equal to 5.5V. And the collector current Ic is equal to 3 mA. Now, let's say we have replaced the transistor and due to that, now the new value of the beta is equal to 50. So, in that case, this collector current Ic will be equal to 50 times 30 uA. That is equal to 1.5 mA. And at that time, if you calculate the value of Vce, then it will be equal to 10 V. minus 1.5 times 1.5 mA. That is equal to 7.75V. That means whenever the value of β becomes 50, at that time the operating point would be somewhere around here. Because at that time the value of Vc is equal to 7.75V and the value of Ic is equal to 1.5 mA. And similarly, let us also see whenever the β becomes 200. So, at that time the collector current Ic will be equal to 200 times 30 micro ampere. That is equal to 6 milliampere. And at that time, the value of Vc will be equal to 10 volt minus 1.5 times 6 milliampere. That is equal to 1 volt. So, at that time the operating point would be somewhere around here. Because at that time, This collector current Ic is equal to 6 mA and the voltage Vce is equal to 1 V. So, this shows the maximum possible change in the operating point due to the change in the temperature or due to the change in the transistor. That means in this fixed-base configuration, even if we set the fixed value of the base current, then also the operating point may vary due to the change in the external parameters. And in the upcoming videos, we will see some other biasing configurations which can provide a relatively stable operating point. And also we will solve some examples on this fixed bias configuration on our second channel. So I hope in this video, you understood the basics of the transistor biasing as well as the fixed bias configuration. So if you have any question or suggestion, do let me know here in the comment section below. If you like this video, hit the like button and subscribe to the channel for more such videos.

Where if we apply the time varying signal as an input, then it amplifies the input signal. But this BJT won't amplify the input signal until we apply the DC power supply. And in fact, The energy supplied by this DC supply is used to amplify the input signal. So, the process of applying this DC voltage source to the BJT is known as the biasing. And if you have followed the previous videos, we had seen that the common emitter configuration of the BJT is frequently used for the signal amplification.

So, for understanding the transistor biasing, we will use the common emitter configuration. Now, whenever The BJT is used as an amplifier, then it is biased in such a way that base emitter junction gets forward biased and the collector base junction gets reversed biased. And once we apply the DC voltage sources to the BJT, then the current and the voltage will establish in the circuit. And as we had seen in the previous video, this voltage Vbe and Ib are the parameters on the input side. While this voltage Vce and Ic.

are the parameters of the output side. And after biasing the BJT in the active region, if we apply the sine wave as an input, then we should get the amplified signal at the output. But whether we will get the proper amplified output or not, that depends on how well the BJT is biased in the active region. So let's understand it through the output characteristics.

So let's say, the BJT is biased in such a way that The VCE is equal to 5V and the collector current IC is equal to 10mA. And this point on this collector curve is known as the operating point or the Q point of the BJT. And the reason it is known as the operating point, because it tells us the operating voltage and the current of the given transistor.

So, this particular operating point tells us that the transistor is biased in such a way that, The base current Ib is equal to 30 uA and the voltage Vc and the Ic are 5V and 10 mA respectively. So, basically this point on the collector curve tells us the operating condition of the transistor. Now, as I said earlier, the operating point should be in the active region.

But in the active region, it could be set anywhere. So, this point could be in the center or it could be over here. Or maybe it could be over here also.

So whenever the operating point is over here and on top of it, whenever we apply the AC signal, then some portion of the amplified signal will get clipped. Because as you can see, this voltage we see cannot go below 0V. Similarly, whenever it is operated at this point, then also some portion of the current will get clipped.

Because the collector current current go below 0 A. And due to that, some portion of this voltage we see will also get clipped. That means whenever the operating point is near saturation or the cut-off region, then it may lead to the non-linear distortion in the output waveform. Also, if you notice over here, when the BJT is operating in this region, then it is operating near the breakdown region. So, the operation of the BJT in this region should be avoided.

On the other end, when the operating point is in the center of the collector curves, then it is possible to amplify the input signal without any kind of distortion. And also in this region, the gain of the BJT is also almost constant. That means for the small signal amplification, the biasing point or the operating point should be in the center region of the collector curves. Now another important aspect of the biasing is the stability of the operating point. Because with temperature, the operating point may get changed.

Because with temperature, the device parameters like the transistor current gain and the reverse saturation current will get changed. So, the designed biasing circuit should provide the temperature stability such that even if there is a change in the temperature, then there is a minimum change in this operating point. And the stability of the operating point is defined by the term stability factor, which indicates the change in the operating point with the change in the temperature.

So, in this video as well as in the upcoming videos, we will see the different biasing techniques for the BJT. And we will compare the stability of the different biasing configurations. So, first of all, let's start with the very simple fixed bias configuration. So, this is the circuit of the fixed bias configuration. Or if I redraw the same circuit, then it can be redrawn like this.

So, as you can see over here, the AC input signal is applied between the base and the emitter terminal, while the output is taken across this collector and the Now, here as we are interested in the DC analysis of the circuit, so we can replace all the capacitors with the open circuit. And then after if you see the equivalent circuit, then it will look like this. Now, the reason it is known as the fixed-base configuration, because for the given power supplies, if we fix the value of this base resistor, then this base current IB will get fixed. And here.

If we mark the voltages, then this voltage VBE is the voltage on the input side, while the voltage VCE is the voltage on the output side. So, first of all, now let's find the expression for this base current IB. So, if you notice over here, the voltage at this node is equal to VBE. So, from this, we can say that this base current IB is equal to this voltage VBB minus VCE. VBE divided by RB.

Now, here the typical value of VBE is equal to 0.7V. So, for the given supply voltage, once we set the value of RB, then this base current IB will get fixed. And once we know the value of this base current, then we can find the value of this collector current and the voltage VCE. Now, we know that this collector current IC can be given as...

So, from the base current Ib, we can find the value of this collector current. Now, if we apply the KVL in this output side, then we can write the expression as Vcc minus voltage across this resistor Rc, let's say that is equal to minus Vrc minus Vce that is equal to zero. Or from this, we can say that this voltage Vce.

is equal to Vcc minus Vrc. That is equal to Vcc minus Ic times Rc. So, in this way, we can also get the value of the Vce.

So, here this voltage Vce and the collector current Ic defines the operating point of the transistor. Now, from the equations if you observe, if the value of Rb or Rc changes, or the value of Vcc changes, then the operating point will also change. So now, let's find out how this network parameters defines the possible values of the operating point. Now, from this expression, we can write this collector current Ic is equal to voltage Vcc minus Vce divided by Rc.

So, this collector current Ic will be maximum whenever this voltage Vce is equal to That means the maximum value of the collector current will be equal to voltage Vcc divided by Rc. That means this collector current will be maximum whenever the voltage Vce is equal to zero. Similarly, whenever this collector current Ic is zero, at that time the value of Vc will be maximum.

So, we can say that voltage Vce max is equal to Vcc. So, from this graph, we can say that whenever the collector current Ic is zero, at that time the value of Vc is maximum. So, if we join these two points, then we will get the possible values of the operating point for the given values of Vcc and the Rc. So, this line is also known as the load line.

Because it is defined by the value of this resistor Rc. Now here, if the value of Ib is changed by varying this Rb, then the Q point will also change. So, for the given value of Vcc and Rc, if the value of Ib is increased, then the Q point will also move upwards.

Similarly, by keeping the Vcc fixed, if we change the value of Rc, then the load line will also change. So here, three different door lines are shown for the different values of the Rc. And as you can see, for the fixed value of the base current, if the load line changes, then the operating point will also shift towards the left-hand side.

And similarly, by keeping the value of Rc fixed, if we change the value of Vcc, then Then the load line will look like this. So here, the different load lines are shown for the different values of Vcc. But let's say, for the given value of Vcc and Rc, we are getting this load line.

And the operating point is also set over here. Now as I said earlier, as the temperature changes, then the operating point will also change. Because with temperature, the value of β or the current gain of the transistor will also change. Or in case, if we replace the transistor, then also the value of β will change. Because if you take the similar transistors, then also they have the different values of β.

So, now let's take one simple example and let's understand how the variation in this β can affect the operating point. So, let's say for a one transistor, the nominal value of the β is equal to 100. And due to the change in the temperature, or due to the replacement of the transistor, this β varies from 50 to 200. And assuming this voltage VBE is not changing with the temperature, this base current IB is set to 30 uA. Now here, let's say the value of Rc is equal to 1.5 kΩ and the value of Vcc is equal to 10 V. So, whenever the β is equal to 100, at that time, This collector current Ic will be equal to 100 times 30 uA.

That is equal to 3 mA. And for that, if you calculate the value of VCE, then it will be equal to 10V-1.5 times 3 mA. That is equal to 5.5V. So, at that time, the operating point would be over here.

That means the VCE is equal to 5.5V. And the collector current Ic is equal to 3 mA. Now, let's say we have replaced the transistor and due to that, now the new value of the beta is equal to 50. So, in that case, this collector current Ic will be equal to 50 times 30 uA.

That is equal to 1.5 mA. And at that time, if you calculate the value of Vce, then it will be equal to 10 V. minus 1.5 times 1.5 mA. That is equal to 7.75V.

That means whenever the value of β becomes 50, at that time the operating point would be somewhere around here. Because at that time the value of Vc is equal to 7.75V and the value of Ic is equal to 1.5 mA. And similarly, let us also see whenever the β becomes 200. So, at that time the collector current Ic will be equal to 200 times 30 micro ampere. That is equal to 6 milliampere.

And at that time, the value of Vc will be equal to 10 volt minus 1.5 times 6 milliampere. That is equal to 1 volt. So, at that time the operating point would be somewhere around here.

Because at that time, This collector current Ic is equal to 6 mA and the voltage Vce is equal to 1 V. So, this shows the maximum possible change in the operating point due to the change in the temperature or due to the change in the transistor. That means in this fixed-base configuration, even if we set the fixed value of the base current, then also the operating point may vary due to the change in the external parameters. And in the upcoming videos, we will see some other biasing configurations which can provide a relatively stable operating point. And also we will solve some examples on this fixed bias configuration on our second channel.

So I hope in this video, you understood the basics of the transistor biasing as well as the fixed bias configuration. So if you have any question or suggestion, do let me know here in the comment section below. If you like this video, hit the like button and subscribe to the channel for more such videos.

Transcript for:Understanding Transistor Biasing Techniques

Transcript for:
Understanding Transistor Biasing Techniques